1. Field of the Invention
The present invention generally relates to a dimming method of Light Emitting Diode (LED). More particularly, the present invention relates to an LED dimming method, which utilizes high impedance node for inputting a dimming control signal but without the usage of an enable pin additionally.
2. Description of the Prior Art
Nowadays, many consumer electronic products contain LCD panel, such as cell phone, PDA, and MP3 player . . . etc. The designer chooses not only the preferred sizes of the LCD panel, but also determines the types of the LCD panel and thereafter the power supply and the back-light circuit thereof in accordance with different applications. Among all kinds of the back-light circuit solutions, white LED is the most adopted solution to portable consumer electronic products.
Generally, the white LED solution may be categorized into the charge pump solution and the inductance-based solution. Herein the charge pump solution is also called switching capacitance solution, which transfers the power from the input terminal to the output terminal by switching the connections between the input terminal with a flying capacitance and between the flying capacitance with the output terminal. The whole switching process is unnecessary of any inductance device. Besides, charge pump devices occupy less volume and benefit in ease of design—the designer usually has no need to change the architecture of the charge pump driving circuitry but only chooses a proper capacitance device according to different demands, hence the charge pump solution is very popular.
In many situations, the LCD panel requires dimming functions. For example, the user may want to brighten a cell phone's or a PDA's screen while under dark environment, or lower the brightness of a cell phone and cut the back-light off after idled for a period of time. A well-known LED dimming method is Pulse Width Modulation (PWM) dimming technique, in which the LED dimming signal is modulated in PWM signal. With further classification, one of the PWM-based dimming methods adjusts the current flowed into the LED by changing the “duty cycle” of the PWM dimming signal. For example, if the duty cycle of the PWM dimming signal is 60%, the current flowed into the LED will be about 60% of the max value, and the brightness of the LED will decrease 60%. Another PWM-based dimming method dims the LED by adjusting the pulse numbers of the PWM signal. For example, the MAX1574 series white LED controller of the MAXIM INC. decreases the brightness of the driving LED with 10% each time after the controller received a pulse of PWM signal. The above-mentioned two types of dimming methods are the most popular, even if they are a little different, but they both use PWM signal to dim LED.
Referring to FIG. 1, FIG. 1 demonstrates a conventional LED driving circuit based on charge pump solution. The LED driving IC 10 includes a charge pump 12, a chip enable/dimming control module 14 and a current sink 16. Herein, the pin CP and CN of the LED driving IC 10 are connected to a transfer-capacitor (fly capacitor) Cf, which couples the electric current from the input terminal VIN to the output terminal VOUT for driving LEDs. The current sink 16 controls and provides stable current flowed through the LEDs while there are no external dimming signals. The chip enable/dimming control module 14 receives dimming control signals from the chip-enable pin EN to control the ON/OFF of the charge pump 12 and the current sink 16, thereby achieving the main object of the LED driving circuit: control the current flowed through the LEDs and then dim the LEDs.
No matter which company's LED driving IC is, most of them need a chip-enable pin as the input node for receiving dimming control signals. If the LED driving IC could receive dimming control signals without any single input pin (chip-enable pin), the pin's numbers could effectively reduced and thereafter the package size of the IC could be reduced, too. In additions, the package cost also could be decreased because of less pins of the IC, and the LED driving circuit with the improved IC will be cheaper and more competitive.